1. Field of the Invention
The present invention relates to soldering tools, particularly those useful for attaching braided lead wires to ceramic metal bus bars on a substrate. A frequent use for the present invention is the solder connection of a lead wire comprising a tinned copper braid to each of a pair of bus bars of ceramic silver composition. The solder connections between tinned copper lead wires to ceramic silver bus bars are used in heated rear windows for automobiles (popularly termed backlights) to provide good electrical connection between a heated circuit interconnecting the pair of bus bars and a voltage source to which the lead wire is adapted for connection. Good electrical connections require a plurality of solder connections at spaced positions along the length of each bus bar so as to minimize the total resistance of the heating circuit and bus bar system for the heated backlight.
In the past, lead wires have been attached to bus bars by applying hand clamps over a lead wire portion applied to an edge portion of a glass sheet to clamp the lead wire to the glass sheet beyond each end of a bus bar to secure the lead wire to the substrate along an edge portion thereof that contains a bus bar. A plurality of solder application points are marked along each bus bar with a flux, and solder is applied by holding a wire of solder at each of the fluxed areas in turn starting from the clamped end of the bus bar and gradually soldering each fluxed area by applying the tip of a soldering tool to a solder wire in each fluxed area in turn to form a pool of solder in the fluxed area. Since it is necessary for the solder to cool into the solid state before a secure bond is made between the lead wire and the bus bar, it is necessary for the operator to remove the soldering tool from the pool of solder. The viscosity of the solder may cause the solder to lift the lead wire and thus lift the lead wire with the soldering tool. This sometimes destroys the solder connection entirely.
It is awkward, if not impossible, for an operator to remove the soldering tool from the vicinity of the fluxed area and still keep the lead wire in alignment with the portion of the bus bar to which it is soldered at any fluxed area. This difficulty has been met by applying additional clamping means at an engagement point on the downstream side of each successive fluxed area in the direction of successive solder application points. However, it would be desirable for an operator to be able to maintain the lead copper braid wire in proper alignment at each solder application point during the time that the pool of solder is being formed from the solder wire and also during the time that the pool is being cooled to a temperature at which a strong bond is obtained between the lead wire and the bus bar without requiring the movement of clamping means in advance of the path of successive solder application points.
A pull test of 25 pounds must be passed before the lead wire-to-bus bar connection is acceptable by customer standards. Hence, the amount of solder that forms a pool and hardens at each solder application point must be sufficient to enable the heated backlight to pass the pull test.
2. Description of Patents of Interest
U.S. Pat. No. 3,467,818 to Ballentine discloses an electrically heated backlight with a pair of bus bars having spaced connections to a pair of lead wires, one providing electrical connection to ground and the other providing electrical connection to a 12 volt electrical system of an automobile. The need for a strong bond between the lead wire and the bus bar is discussed in this patent. The solder connections are spaced from one another and the lead wire extends loosely between solder connections over the entire length of the bus bar to which it is bonded by solder. The reason for extending the lead wires loosely between the spaced solder connections is to compensate for any difference in thermal expansion between each lead wire and its aligned bus bar that results from changes in temperature of the window during its use, particularly when current is applied to the heating circuit interconnecting the bus bars to heat a window that is either fogged or iced.
Unless the lead wire is held in proper alignment with the ceramic silver bus bar of the heated backlight of the automobile during the melting of the solder wire to form a pool of solder and during the time that the pool of solder cools to form a solid spot of solder, the solder connection may be either weakened or broken entirely. Under such circumstances, the solder connections will fail to pass the pull test.
U.S. Pat. No. 3,480,759 to Sachs et al shows a soldering tool pivotally supported from a relatively massive stand. The massive stand is difficult to move from one solder application point to another and is not suggested as a hold-down to secure workpieces together during soldering.
U.S. Pat. No. 3,558,854 to Siegel et al discloses a miniature soldering gun mounted in movable relation to a pencil-type grip by a movable spring-biased electrical connector assembly inside the pencil grip. This apparatus does not include any hold-down device for the workpieces.
U.S. Pat. No. 3,706,871 to Witherow et al also discloses a self-supporting soldering tool pivotally mounted on a base. The base of this device is too massive to suggest its use as a movable hold-down for workpieces.
U.S. Pat. No. 3,673,664 to Niese discloses a solder application point locating device that comprises a removable clip that holds a lead wire against a bus bar and has a plurality of apertures that coincide with solder application points along the length of the device. The device holds the lead wire against the bus bar to guide the locations for applying solder through the apertures. The separate device must be removed after the solder hardens. The device operates entirely independently of the soldering apparatus and it would be beneficial to the backlight soldering art to avoid the use of such a device.
U.S. Pat. No. 3,685,714 to Moyer et al discloses apparatus to solder flexible, braided lead wires to strip electrodes fused to a backlight. This apparatus includes conveyor means to automatically locate the backlights relative to special soldering equipment and is characterized by complicated control circuitry and mechanisms to coordinate glass sheet positioning with the application of solder. Such mechanical equipment requires maintenance and it would be convenient to avoid the need for such complicated equipment.
U.S. Pat. No. 3,940,046 to Fern discloses apparatus that automatically applies molten solder to connect a wire tab to wire elements that form a windshield antenna and to periodically remove excess solder from the soldering iron tip. Such apparatus is not suitable for use in making heated backlights.